The present invention relates to tools for applying mastic and especially to flat finisher tools for applying mastic to drywall joints.
Flat finisher tools are well known in the housing construction and drywall industries, and are used to apply a coating of mastic to conceal the joints between drywall panels. An example of a typical flat finisher tool is provided by U.S. Pat. No. 2,984,857, which is incorporated herein by reference. Conventional flat finisher tools consist of a container for holding a supply of mastic, with an opening for dispensing the mastic. A flexible trowelling bar is attached to the tool for shaping and smoothing the surface of the applied mastic. As the mastic is dispensed, the trowelling bar flexes over the applied mastic to form a concave curve that shapes the mastic into a layer that is thick in the center and is feathered or thin at the edges to blend into the surface of the drywall.
The curvature of the trowelling bar and, therefore, the thickness of the applied mastic, is indirectly controlled by a cam that regulates the pressure exerted by a leaf spring on the trowelling bar. Actuating the cam increases the pressure on the leaf spring, which increases the pressure on the trowelling bar to reduce its curvature and produce a thinner layer of mastic. However, the leaf spring often becomes distorted after repeated use, which reduces its effectiveness in controlling the curvature of the trowelling bar.
The trowelling bar typically comprises a normally flat blade mounted in a brass rod that serves as a blade guide. Because the curvature of the trowelling bar depends on its ability to flex over the mastic, numerous slots are cut perpendicular to the longitudinal axis of the brass rod to increase its flexibility and ensure that the blade guide will arch or curve properly as it passes over the mastic. Forming these slots requires extensive machining and adds to the expense of manufacturing the trowelling bar. In addition, the brass trowelling bar is relatively heavy, making the tool more difficult to wield and more tiring to use.
Thus, there is a need for a flat finisher tool that reduces or eliminates the reliance on a leaf spring to control the curvature of the trowelling bar. In addition, there is a need for a trowelling bar that is simpler and less expensive to manufacture, and that is made of lighter weight materials.